An ideal FM detector has a linear relationship between the instantaneous frequency deviation and the output of the detector. Moreover, in the ideal case the output of the detector for an un-modulated carrier would be known exactly. Without loss of generality this known value can be considered to be zero. This simply means that a positive frequency deviation will result in a positive output from the FM detector and a negative frequency deviation will result in a negative output (or vice versa). Since binary information is preferably transmitted by mapping a zero to a negative frequency deviation and a one to a positive frequency deviation, a hard decision can then simply be made by considering the sign of the output of the FM detector. If instead soft information was to be used, not only the sign, but also the actual value of the output of the detector would be used. This is also the case if non-binary alphabets were to be employed.
In practice there will be frequency offsets in the received signal, which are due to various imperfections at both the transmitter and the receiver. This means that the output from the FM detector is not zero for an un-modulated carrier, and therefore zero cannot be used as a threshold to make a decision at the output of the FM detector. Thus, a proper choice of threshold has to be found. Since the frequency offset is not known beforehand and typically is not even constant through the reception of a packet, it has to be found and adjusted dynamically. There are several known ways to do this.
If the received data itself is DC-free, the DC-level can be found by integrating/low-pass filtering the received signal.
However, this approach inherently has the property that one has to trade accuracy for settlement time. To overcome this trade-off to some extent, one can use a smaller time-constant during the initial part of the packet to get a rapid but coarse DC-estimate, and then shift to a larger time-constant to obtain a smoother and more accurate DC-estimate once the coarse estimate is considered as completed.
Another known method for estimation of the DC-level is to make use of a circuit which clamps the maximum and the minimum values of the received signal. The average of these two values is then taken as the DC-level. This has the advantage of being very fast because the maxima and the minima of the signal are present at an early stage. For DC-estimation based on max-min, it is essential that the signal does not contain any spikes or similar phenomena, since the max/min detector will react to these. Moreover, since the DC-level may vary during the reception of a packet, the maximum and minimum values have to decay (towards the estimated DC), in order to guarantee that such variations actually can be followed. The rate of this decay will be a trade-off between being able to follow a varying DC-level and having as stable DC estimate as possible.